Glass for a light filter and a light filter

ABSTRACT

Glass for a light filter capable of preventing variation of refractive index in a band-pass filter has a coefficient of thermal expansion within a range from 90×10 −7 /° C. within a temperature range from −20° C. to +70° C. and, preferably, Young&#39;s modulus of 75 GPa or over and Vickers hardness of 550 or over, and light transmittance for plate thickness of 10 mm of 90% or over within a wavelength range from 950 nm to 1600 nm.

RELATED PATENT APPLICATION

This application is a continuation-in-part of our prior application U.S.Ser. No. 09/550,879 filed Apr. 17, 2000.

BACKGROUND OF THE INVENTION

This invention relates to glass for a light filter and also to a lightfilter using glass.

There are light filters which cut or pass light of a specific wavelengthand there are also light filters which reduce intensity of light withoutdepending upon wavelength. The former includes a band-pass filter whichpasses only a specific wavelength, a notch pass filter which cuts aspecific wavelength and high-pass and low-pass filters which pass onlywavelengths shorter or longer than a specific wavelength. The latterincludes an ND filter.

Light filters can be classified also into an absorption type filter andan interference type filter. A representative absorption type filter isthe ND filter and a representative interference type filter is theband-pass filter. A substrate made of plastic is used for absorptiontype filters such as those for photography. Since a substrate for lightfilters which are subject to a strong laser beam requires durability andheat resistance property, amorphous glass is exclusively employed forsuch substrate.

The band-pass filters are made by forming, on a substrate made of, e.g.,glass, a multi-layer film of dielectric by alternately laminating an Hlayer of a dielectric thin film having a high refractive index and an Llayer of a dielectric thin film having a low refractive index.

In a band-pass filter which is used for the WDM(wavelength divisionmultiplexing) optical communication system, temperature stability of thecenter wavelength of the band poses a problem when a narrow band widthfor passing wavelengths is set for applying the band-pass filter to awavelength of a higher density. More specifically, the band-pass filteris a sensitive element in which the center frequency of the band varieseven with a slight variation in temperature and, therefore, temperaturecompensation should be made by a temperature controller when theband-pass filter is used. Such temperature controller, however, cannotactually be employed because of limitation in the space where theband-pass filter is located. The temperature stability has become amatter of increasing importance since it is necessary to reduce the bandwidth as the amount of light information increases.

In the past, amorphous glass has been used as a substrate for theband-pass filter. This prior art substrate is not sufficient in itscompressive stress to the film and its durability since its thermalexpansion property and mechanical strength are not sufficiently high.Further, amorphous glass has a low surface hardness. Thus, amorphousglass cannot sufficiently satisfy the demands for a substrate for alight filter, particularly a substrate for a band-pass filter.

It is, therefore, an object of the invention to provide a materialsuitable for a substrate for a light filter which has eliminated theabove described disadvantages of the prior art substrate and has athermal expansion property which is sufficient for avoiding variation inthe refractive index at a temperature at which a filter formed with amono-layer or multi-layer film is used (i.e., having a high coefficientof thermal expansion and thereby imparting compressive stress to thefilm to improve temperature stability of the refractive index of thefilm) and also has a mechanical property which imparts sufficientdurability to the filter and further has excellent light transmittance.

It is, another object of the invention to provide a light filter made ofsuch substrate.

SUMMARY OF THE INVENTION

Accumulated studies and experiments made by the inventors of the presentinvention for achieving the above described objects of the inventionhave resulted in the finding, which has led to the present invention,that glass having a coefficient of thermal expansion, mechanicalstrength and light transmittance within specific ranges is suitable forachieving these objects of the invention.

According to the invention, there is provided glass for a light filterhaving a coefficient of thermal expansion within a range from 90×10⁻⁷/°C. to 120×10⁻⁷/° C. within a temperature range from −20° C. to +70° C.and having a composition which comprises, in weight percent:

one or more ingredients selected from the group consisting of SiO₂, B₂O₃and P₂O₅ in the total amount of 35-55%, wherein the upper limit of SiO₂is 41.5%;

one or more ingredients selected from the group consisting of TiO_(2,)La₂O₃, ZrO₂, Nb₂O₅, Ta₂O₅, WO₃ and Y₂O₃ in the total amount of 20-45%,wherein TiO₂ up to 30% is included and ZrO₂ is included within a rangefrom 0 to 5%;

one or more ingredients selected from the group consisting of MgO, CaO,SrO, BaO and ZnO in the total amount of 3-20%;

Na₂O within a range from 0 to 14.5%; and

one or both of Sb₂O₃ and As₂O₃ in the total amount of 0-1%,

said glass being substantially free of Al₂O₃, CdO and PbO.

In one aspect of the invention, there is provided glass for a lightfilter having a coefficient of thermal expansion within a range from90×10⁻⁷/° C. to 120×10⁻⁷/° C. within a temperature range from −20° C. to+70° C. and having a composition which comprises, in weight percent:

one or more ingredients selected from the group consisting of SiO₂, B₂O₃and P₂O₅ in the total amount of 35-55%, wherein the upper limit of SiO₂is 41.5%;

TiO₂ up to 30%;

ZrO₂ within a range from 0 to 5%;

one or more ingredients selected from the group consisting of MgO, CaO,SrO, BaO and ZnO in the total amount of 3-20%;

one or more ingredients selected from the group consisting of Li₂O, Na₂Oand K₂O in the total amount of 5-30%, wherein Na₂O is included within arange from 0 to 14.5%; and

one or both of Sb₂O₃ and As₂O₃ in the total amount of 0-1%,

said glass being substantially free of Al₂O_(3,) CdO and PbO.

In another aspect of the invention, there is provided glass for a lightfilter having a coefficient of thermal expansion within a range from90×10⁻⁷/° C. to 120×10⁻⁷/° C. within a temperature range from −20° C. to+70° C. and having a composition which comprises, in weight percent:

one or more ingredients selected from the group consisting of SiO₂, B₂O₃and P₂O₅ in the total amount of 35-55%, wherein the upper limit of SiO₂is 41.5%;

one or more ingredients selected from the group consisting of TiO_(2,)La₂O₃, ZrO₂, Nb₂O_(5,) Ta₂O₅, WO₃ and Y₂O₃ in the total amount of20-45%; wherein TiO₂ up to 30% is included and ZrO₂ is included within arange from 0 to 5%;

one or more ingredients selected from the group consisting of MgO, CaO,SrO, BaO and ZnO in the total amount of 3-20%;

Na₂O within a range from 0 to 14.5%; and

one or both of Sb₂O₃ and As₂O₃ in the total amount of 0-1%, said glassbeing substantially free of Al₂O_(3,) CdO and PbO.

In another aspect of the invention, there is provided glass for a lightfilter having a coefficient of thermal expansion within a range from90×10⁻⁷/° C. to 120×10⁻⁷/° C. within a temperature range from −20° C. to+70° C. and having a composition which comprises, in weight percent:

one or more ingredients selected from the group consisting of SiO₂, B₂O₃and P₂O₅ in the total amount of 35-55%, wherein the upper limit of SiO₂is 41.5%;

one or more ingredients selected from the group consisting of TiO_(2,)La₂O₃, ZrO₂, Nb₂O_(5,) Ta₂O₅, WO₃ and Y₂O₃ in the total amount of20-45%, wherein TiO₂ up to 30% is included;

one or more ingredients selected from the group consisting of MgO, CaO,SrO, BaO and ZnO in the total amount of 3-20%;

one or more ingredients selected from the group consisting of Li₂O, Na₂Oand K₂O in the total amount of 5-30%, wherein Na₂O is included within arange from 0 to 14.5%; and

one or both of Sb₂O₃ and As₂O₃ in the total amount of 0-1%,

said glass being substantially free of CaO and CdO.

In another aspect of the invention, there is provided glass for a lightfilter having a coefficient of thermal expansion within a range from90×10⁻⁷/° C. to 120×10⁻⁷/° C. within a temperature range from −20° C. to+70° C. and having a composition which comprises, in weight percent:

one or more ingredients selected from the group consisting of SiO₂, B₂O₃and P₂O₅ in the total amount of 35-55%, wherein the upper limit of SiO₂is 41.5%;

one or more ingredients selected from the group consisting of TiO_(2,)La₂O₃, ZrO₂, Nb₂O₅, Ta₂O ₅, WO₃ and Y₂O₃ in the total amount of 20-45%wherein Tio₂ up to 30% is included;

one or more ingredients selected from the group consisting of MgO, CaO,SrO, BaO and ZnO in the total amount of 3-20%;

one or more ingredients selected from the group consisting of Li₂O, Na₂Oand K₂ in the total amount of 5-30%, wherein Na₂O is included within arange from 0 to 14.5%; and

one or both of Sb₂O₃ and As₂O₃ in the total amount of 0-1%,

said glass being substantially free of CaO and CdO.

In one aspect of the invention, the glass has Young's modulus of 75 GPaor over.

In another aspect of the invention, the glass has Vickers hardness of550 or over.

In another aspect of the invention, light transmittance for platethickness of 10 mm is 90% or over within a wavelength range from 950 nmto 1600 nm.

In another aspect of the invention, there is provided a light filterwhich is made by forming a dielectric film on the above described glass.

In still another aspect of the invention, there is provided a lightfilter which is made by forming a dielectric film on glass having alarger coefficient of thermal expansion than dielectric whichconstitutes the dielectric film.

DETAILED DESCRIPTION OF THE INVENTION

Reasons for limiting the thermal expansion property, Young's modulus,Vickers hardness, light transmittance and composition of the glass for alight filter according to the invention will be described below. Thecomposition of the glass is expressed on the oxide basis (in weightpercent).

Description will be made first about thermal expansion property. Asdescribed previously, the temperature stability of the center wavelengthof the band is very important and glass-ceramics having a largercoefficient of thermal expansion than a material which constitutes thefilm is required. The studies and experiments conducted by the inventorsof the present invention have resulted in the finding that, if thecoefficient of thermal expansion within the temperature range from −20°C. to +70° C. is 90×10⁻⁷/° C. or over, sufficient compression stress canbe imparted to the film within a temperature range in which the glass isused as a band-pass filter and that, if the coefficient of thermalexpansion exceeds 120×10⁻⁷/° C., difference in the coefficient ofthermal expansion between the substrate and the filter becomes so largethat problems such as separation of the film from the substrate takeplace. A preferable range of the coefficient of thermal expansion is95×10⁻⁷/° C. to 115×10⁻⁷/° C. and a more preferable range thereof is105±5×10⁻⁷/° C.

In the band-pass filter, the temperature stability of the centerwavelength depends to some extent on a refractive index temperaturecoefficient of a dielectric which constitutes the thin film and, to alarger extent than that, on a coefficient of thermal expansion of thesubstrate. This is because refractive index is determined by a filmatomic density of the thin film. That is, the higher the film atomicdensity of the thin film is, the smaller becomes variation caused by thetemperature of the center frequency. The film atomic density of the thinfilm is greatly influenced by the coefficient of thermal expansion ofthe substrate for the light filter on which the thin film is formed.More specifically, the temperature of the substrate during the filmforming process becomes about 200° C. and the substrate thereby isconsiderably expanded. The thin film is formed on this expandedsubstrate and, as the substrate is cooled, the thin film is subjected tocompressive stress due to difference in the coefficient of thermalexpansion between them. As a result, the film atomic density of the thinfilm increases and the refractive index thereby increases. As thecoefficient of thermal expansion of the substrate increases, thecompressive stress applied to the dielectric thin film formed on thesubstrate increases with the result that variation in the refractiveindex due to temperature at which the filter is used decreases. For thisreason, it is desirable to set the coefficient of thermal expansion ofthe glass at a larger value than the coefficient of thermal expansion ofthe dielectric thin film.

Having regard to severe conditions in which the band-pass filter isused, strength against mechanical deformation, i.e., Young's modulus andVickers hardness of the glass, in addition to the above describedproperties, cannot be ignored.

As regards Young's modulus, since the substrate is processed to a smallchip (2 mm×2 mm×2 mm or below) after forming of a thin film, a highYoung's modulus and a high strength are required for the substrate.Having regard to such subsequent processing, the glass should preferablyhave Young's modulus of 75 GPa or over and Vickers hardness of 550 orover.

As regards light transmittance, if light transmittance is low,inconveniences such as reduction in the signal-to-noise ratio will takeplace in producing signals. It is therefore desirable that lighttransmittance should be as large as possible and it has been found thatlight transmittance of 90% at the minimum is necessary. The wavelengthrange used for the band-pass filter is 950 nm to 1600 nm and lighttransmittance of 90% or over for plate thickness of 10 mm is requiredfor this wavelength range. As to light transmittance within thiswavelength range, light transmittance should preferably be 93% or overand, more preferably, 95% or over.

Reasons for limiting the composition range of the base glass asdescribed above will now be described.

The SiO₂, B₂O₃ and P₂O₅ ingredients are glass forming ingredients. Ifthe total amount of one or more of these ingredients is below 35%,chemical durability of the glass deteriorates and Young's modulus andVickers hardness decrease whereas if the total amount of theseingredient exceeds 55%, difficulty arises in melting and forming thebase glass and, moreover, high thermal expansion property of the glasscannot be achieved.

The TiO₂, La₂O₃, ZrO₂, Nb₂O₅, Ta₂O₅, WO₃ and Y₂O₃ ingredients areimportant ingredients which can impart high thermal expansion property,high Young's modulus and high Vickers hardness to the glass containing arelatively large amount of alkali ingredients i.e., Li, Na and K). Ifthe total amount of one or more of these ingredients is below 20%, theseeffects cannot be obtained whereas if the total amount exceeds 45%,difficulty arises in melting and forming of the glass and resistance todevitrification during forming of the glass deteriorates.

The Li₂O, Na₂O and K₂O ingredients are effective for improving themelting property of the glass and increasing high thermal expansionproperty while maintaining high Young's modulus. If the total amount ofone or more of these ingredients is below 5%, these effects cannot beachieved whereas if the total amount exceeds 30%, chemical durability aswell as Young's modulus and Vickers hardness deteriorate.

The MgO, ZnO, CaO, BaO and SrO ingredients are effective for improvingthe melting property of the glass and increasing high thermal expansionproperty while maintaining high Young's modulus. If the total amount ofone or more of these ingredients is below 3%, these effects cannot beachieved whereas if the total amount exceeds 20%, the melting andforming property of the glass deteriorate.

The Sb₂O₃, and As₂O₃ ingredients may be added as a refining agent inmelting the glass. Addition of the total sum of one or both of theseingredients up to 1% will suffice. A preferable total sum is 0.5% orless.

To such an extent as not to impair the required properties of the glass,SnO, CuO, CoO, NiO, Fe₂O₃, CeO and/or MnO may be added up to 5%.

As to PbO, this ingredient is undesirable from the standpoint ofprotecting the environment and use of this ingredient should be avoidedto the maximum extent possible.

For manufacturing the glass for a light filter according to theinvention, the base glass having the above described composition ismelted and formed to a predetermined shape and, annealed if necessary.By lapping and polishing the glass thus obtained by a conventionalmethod, glass for a light filter having surface roughness (Ra)(arithmetic mean roughness) within a range from 1.0 Å to 5.0 Å isprovided.

The glass according to the invention is suitable for an interferencetype light filter in which a multi-layer film of dielectric is formed ona glass substrate, particularly, for a band-pass filter having adielectric multi-layer film formed by alternately laminating adielectric thin film (H layer) having a high refractive index and adielectric thin film (L layer) having a low refractive index.

As the dielectric, inorganic oxides such as TiO₂, Ta₂O₂, Nb₂O₅ and SiO₂may preferably be used. In a band-pass filter used for the wavelengthrange from 950 nm to 1600 nm, combinations of TiO₂/SiO₂, Ta₂O₂/SiO₂ andNb₂O₅/SiO₂ may preferably be used as combinations of the H layer and theL layer.

The light filter of the present invention can be provided by forming adielectric thin film on the surface of the glass substrate. For formingthe thin film, deposition, RF ion plating, magnetron spattering, plasmaion plating etc. may be employed. Among them, deposition is particularlypreferable.

EXAMPLES

Examples of the invention will now be described. Tables 1, 2 and 3 showExamples No. 1 to No. 8 of the glass for a light filter according to theinvention and a comparative example of a prior art glass substrate for alight filter with respect to their composition, coefficient of thermalexpansion, Young's modulus, Vickers hardness and light transmittance.

TABLE 1 Examples 1 2 3 SiO₂ 41.39 39.39 41.39 (Wt. %) B₂O₃ 2.00 P₂O₅La₂O₃ TiO₂ 30.00 30.00 26.00 ZrO₂ 2.10 Nb₂O₅ Ta₂O₅ WO₃ Y₂O₃ 4.00 MgO 0.5ZnO BaO 3.50 3.50 2.50 SrO 0.5 Li₂O 2.00 2.00 2.00 Na₂O 14.10 14.1013.00 K₂O 9.00 9.00 8.00 Sb₂O₃ 0.01 0.01 As₂O₃ Coefficient of thermal110 100 115 expansion (× 10⁻⁷/° C.) (−20° C. to +70° C.) Young's modulus(GPa) 85 87 114 Vickers hardness (Hv) 550 580 640 Light transmittance(%) 99 99 97 (950-1600 nm)

TABLE 2 Examples 4 5 6 SiO₂ 17.10 20.00 28.60 (Wt. %) B₂O₃ 21.50 10.0010.00 P₂O₅ 8.60 La₂O₃ 17.00 12.00 10.00 TiO₂ 7.60 5.00 7.60 ZrO₂ 4.504.50 4.50 Nb₂O₅ 7.90 7.90 7.90 Ta₂O₅ 7.00 WO₃ 2.60 Y₂O₃ 5.00 MgO 1.001.00 1.00 ZnO 1.00 BaO 2.00 10.00 SrO 14.80 11.80 4.80 Li₂O 8.50 8.508.50 Na₂O K₂O Sb₂O₃ 0.10 0.10 As₂O₃ 0.10 Coefficient of thermal 90 95 96expansion (× 10⁻⁷/° C.) (−20° C. to +70° C.) Young's modulus (GPa) 117110 120 Vickers hardness (Hv) 680 630 670 Light transmittance (%) 97 9696 (950-1600 nm)

TABLE 3 Examples Comparative 7 8 Example 1 SiO₂ 40.00 20.00 47.00 (Wt.%) B₂O₃ 10.00 7.10 P₂O₅ 8.60 1.39 La₂O₃ 15.00 TiO₂ 7.60 25.00 ZrO₂ 4.505.00 Nb₂O₅ 7.90 Ta₂O₅ WO₃ 2.00 Y₂O₃ MgO 1.00 PbO = 9.70 ZnO 2.00 0.506.10 BaO 2.00 2.00 10.00 SrO 10.80 1.00 Li₂O 8.50 4.00 Na₂O 12.10 6.00K₂O 9.00 14.00 Sb₂O₃ 0.10 0.01 0.10 As₂O₃ Coefficient of thermal 95 11088 expansion (× 10⁻⁷/° C.) (−20° C. to +70° C.) Young's modulus (GPa)120 85 70 Vickers hardness (Hv) 680 550 480 Light transmittance (%) 9799 99 (950-1600 nm)

For manufacturing the glass of the above described Examples No. 1 to No.8, materials including oxides, carbonates and nitrates were weighed andmixed and molten in a conventional melting apparatus at a temperaturewithin the range from about 1250° C. to 1350° C. The molten glass wasstirred to homogenize it and thereafter formed into a predeterminedshape and annealed to provide a formed glass. Then, the formed glass waslapped with diamond pellets of 800-2000# for 5 minutes to 30 minutes andwas finally polished with cerium oxide polishing grains having anaverage diameter of 0.02 μm to 3 μm for 30 minutes to 60 minutes.Surface roughness (Ra) (roughness at the center line) was 5 Å or below.

Comparing Examples No. 1 to No. 8 with Comparative Example 1, thecoefficient of thermal expansion of the prior art glass substrate for alight filter was 88×10⁻⁷/° C. which was not sufficient for impartingcompressive stress to a formed film. The prior art glass substrateexhibited also low Young's modulus of 70 GPa and Vickers hardness of480. In contrast, the glass of the present invention had a sufficientcoefficient of thermal expansion for imparting compressive stress to theformed film and also sufficient Young's modulus and Vickers hardnesswhich shows that the glass of the present invention is suitable for asubstrate for a light filter.

Interference type light filters which were provided by formingmulti-layer films of TiO₂/SiO₂, Ta₂O₂/SiO₂ and Nb₂O₅/SiO₂ on the glasssubstrates of the above described Examples had an excellent temperaturestability of its center wavelength and were found to be most suitablefor a band-pass filter for optical communication.

As described in the foregoing, according to the invention, thedisadvantages of the prior art substrates are overcome and glasssubstrates for a light filter having an excellent temperature stabilityof a center wavelength are provided. These features, i.e., high lighttransmittance, high thermal expansion property, high Young's modulus andhigh Vickers hardness, are suitable for an interference type filter,particularly a band-pass filter and are most suitable for WDM and DWDM(density wavelength division multiplexing) in optical communicationsystems. Further, the band-pass filter elements which are provided byforming multi-layer dielectric films of TiO₂/SiO₂, Ta₂O₂/SiO₂ andNb₂O₅/SiO₂ on the glass substrates of the invention have an excellenttemperature stability of the center wavelength and can be used not onlyfor optical communication systems on the ground but also for space-basedsatellites.

1. Glass for a light filter having a coefficient of thermal expansionwithin a range from 90×10⁻⁷/° C. to 120×10⁻⁷/° C. within a temperaturerange from −20° C. to +70° C. and having a composition which comprises,in weight percent: one or more ingredients selected from the groupconsisting of SiO₂, B₂O₃ and P₂O₅ in the total amount of 35-55%, whereinthe upper limit of SiO₂ is 41.5%; one or more ingredients selected fromthe group consisting of TiO₂, La₂O₃, ZrO₂, Nb₂O₅, Ta₂O₅, WO₃ and Y₂O₃ inthe total amount of 20-45%, wherein TiO₂ is included within a range from0 to 30% and ZrO₂ is included within a range from 0 to 5%; one or moreingredients selected from the group consisting of MgO, SrO, BaO and ZnOin the total amount of 3-20%; Na₂O within a range from 0 to 14.5%; andone or both of Sb₂O₃ and As₂O₃ in the total amount of 0-1%, said glassbeing substantially free of Al₂O₃, CdO, CaO and PbO.
 2. Glass as definedin claim 1 which has Young's modulus of 75 GPa or over.
 3. Glass asdefined in claim 1 which has Vickers hardness of 550 or over.
 4. Glassas defined in claim 1 wherein light transmittance for plate thickness of10 mm is 90% or over within a wavelength range from 950 nm to 1600 nm.5. A light filter which is made by forming a dielectric film on glass asdefined in claim
 1. 6. Glass for a light filter having a coefficient ofthermal expansion within a range from 90×10⁻⁷/° C. to 120×10⁻⁷/° C.within a temperature range from −20° C. to +70° C. and having acomposition which comprises, in weight percent: one or more ingredientsselected from the group consisting of SiO₂, B₂O₃ and P₂O₅ in the totalamount of 35-55%, wherein the upper limit of SiO₂ is 41.5%; TiO₂ withina range from 0 to 30%; ZrO₂ within a range from 0 to 5%; one or moreingredients selected from the group consisting of MgO, SrO, BaG and ZnOin the total amount of 3-20%; one or more ingredients selected from thegroup consisting of Li₂O, Na₂O and K₂O in the total amount of 5-30%,wherein Na₂O is included within a range from 0 to 14.5% and one or bothof Sb₂O₃ and As₂O₃ in the total amount of 0-1%, said glass beingsubstantially free of Al₂O₃, CdO, CaO and PbO.
 7. Glass as defined inclaim 6 which has Young's modulus of 75 GPa or over.
 8. Glass as definedin claim 6 which has Vickers hardness of 550 or over.
 9. Glass asdefined in claim 6 wherein light transmittance for plate thickness of 10mm is 90% or over within a wavelength range from 950 nm to 1600 nm. 10.A light filter which is made by forming a dielectric film on glass asdefined in claim
 6. 11. Glass for a light filter having a coefficient ofthermal expansion within a range from from 90×10⁻⁷/° C. to 120×10⁻⁷/° C.within a temperature range from −20 ° C. to +70° C. and having acomposition which comprises, in weight percent: one or more ingredientsselected from the group consisting of SiO₂, B₂O₃ and P₂O₅ in the totalamount of 35-55%, wherein the upper limit of SiO₂ is 41.5%; one or moreingredients selected from the group consisting of TiO₂, La₂O₃, ZrO₂,Nb₂O₅, Ta₂O₅, WO₃ and Y₂O₃ in the total amount of 20-45%, wherein TiO₂is included within a range from 0 to 30%; one or more ingredientsselected from the group consisting of MgO, SrO, BaO and ZnO in the totalamount of 3-20%; one or more ingredients selected from the groupconsisting of Li₂O, Na₂O and K₂O in the total amount of 5-30%, whereinNa₂O is included within a range from 0 to 14.5%; and one or both ofSb₂O₃ and As₂O₃ in the total amount of 0-1%, said glass beingsubstantially free of CaO and CdO.
 12. Glass as defined in claim 11which has Young's modulus of 75 GPa or over.
 13. Glass as defined inclaim 11 which has Vickers hardness of 550 or over.
 14. Glass as definedin claim 11 wherein light transmittance for plate thickness of 10 mm is90% or over within a wavelength range from 950 nm to 1600 nm.
 15. Glassas defined in claim 11 which is substantially free of PbO.
 16. A lightfilter which is made forming a dielectric film on glass as defined inclaim 11.